Deriving precise parameters for cool solar-type stars

Abstract

Context. Temperature, surface gravity, and metallicitity are basic stellar atmospheric parameters necessary to characterize a star. There are several methods to derive these parameters and a comparison of their results often shows considerable discrepancies, even in the restricted group of solar-type FGK dwarfs. Aims. We want to check the differences in temperature between the standard spectroscopic technique based on iron lines and the infrared flux method (IRFM). We aim to improve the description of the spectroscopic temperatures especially for the cooler stars where the differences between the two methods are higher, as presented in a previous work. Methods. Our spectroscopic analysis was based on the iron excitation and ionization balance, assuming Kurucz model atmospheres in LTE. The abundance analysis was determined using the code MOOG. We optimized the line list using a cool star (HD 21749) with high resolution and high signal-to-noise spectrum, as a reference in order to check for weak, isolated lines. Results. We test the quality of the new line list by re-deriving stellar parameters for 451 stars with high resolution and signal-tonoise HARPS spectra, that were analyzed in a previous work with a larger line list. The comparison in temperatures between this work and the latest IRFM for the stars in common shows that the differences for the cooler stars are significantly smaller and more homogeneously distributed than in previous studies for stars with temperatures below 5000 K. Moreover, a comparison is presented between interferometric temperatures with our results that shows good agreement, even though the sample is small and the errors of the mean differences are large. We use the new line list to re-derive parameters for some of the cooler stars that host planets. Finally, we present the impact of the new temperatures on the [Cr /Cr ] and [Ti /Ti ] abundance ratios that previously showed systematic trends with temperature. We show that the slopes of these trends for the cooler stars become drastically smaller.

SWEET-Cat: A catalogue of parameters for Stars With ExoplanETs

“…Herefore, the 2002 version of MOOG 4 (Sneden 1973), a grid of ATLAS plane-parallel model atmospheres (Kurucz 1993) and the iron linelist of Sousa et al (2008) are used. For stars cooler than 5200 K (as initially derived with the Sousa et al line list) we re-derived and adopted the parameters using the line list of Tsantaki et al (2013), specially suitable for cool stars. Stellar masses and their errors were computed with the corrected calibration of Torres et al (2010) as discussed in Sect.…”

“…These effects are difficult to quantify, but can be at least partially solved when using appropriate line-lists and methodologies (see e.g. Tsantaki et al 2013;Mortier et al 2013b). …”

CNO behaviour in planet-harbouring stars

“…The stellar parameters used in this study were taken from Sousa et al (2008Sousa et al ( , 2011a, Tsantaki et al (2013) and González Hernández et al (2010. All stellar parameters used were derived by measuring equivalent widths of Fe I and Fe II lines using the code ARES (Sousa et al 2007).…”

“…All the atmospheric parameters, T eff , log g, ξ t and [Fe/H] were taken, as already mentioned, from Sousa et al (2008Sousa et al ( , 2011a, Tsantaki et al (2013) and González Hernández et al (2010. The adopted solar abundances for nitrogen and iron were log (N) = 8.05 dex and log (Fe) = 7.47 ).…”

Line-depth-ratio temperatures for the close binary ν Octantis: new evidence supporting the conjectured circumstellar retrograde planet

“…As a consequence, many methods have been devised to measure it. In their comparison of two methods, one based on iron excitation and ionization balance and the other the infrared flux method, Tsantaki et al (2013) list several others that include interferometry (using the relationship between diameter and luminosity), photometrically derived colour indices (which have different dependencies on temperature), and line properties such as the Hα wings, spectral synthesis strategies and, the method we will employ, line-depth ratios (LDRs). Gray (1992) also discusses various methods, including LDRs.…”